It is already known to concentrate an aqueous polytetrafluoroethylene dispersion by adding a surfactant to the dispersion, thereafter heating the dispersion to form a transparent aqueous solution as an upper layer and to concentrate polymer particles as contained in a lower aqueous layer, and removing the upper layer by decantation. The surfactant used is an ethylene oxide adduct of an alkylphenol (polyoxyethylene alkyl phenyl ether). However, the concentrated dispersion has many problems, such as those given below, for use in impregnating fibrous base materials.
(1) In the impregnating-baking step, the surfactant thermally decomposes, converting into a harmful organic aromatic compound (such as benzene, toluene or xylene) as an exhaust gas to cause air pollution.
(2) The surfactant partially undergoes thermal decomposition to produce a tarlike substance, which accumulates on the inner wall of the baking furnace of the impregnating-baking apparatus. The substance falls onto or adheres to the fibrous base material passing through the furnace to lower the yield from the viewpoint of quality.
(3) The surfactant, which is difficult to thermally decompose or dissipate, partly remains in the fibrous base material after baking, assuming a brown color to seriously impair the hand of the impregnated material.
(4) Because the remaining portion of the surfactant is carbonized, use of the impregnated material in preparing high-frequency printed boards entails impaired electric characteristics.
When the dispersion is used as a battery binder, the surfactant is left undissipated almost entirely under the drying condition (about 250 to about 300.degree. C.) for the electrode material, remaining in the electrode material and contributing to impaired performance.
To solve these problems, aqueous polytetrafluoroethylene dispersion compositions are known in which an ethoxylated aliphatic alcohol (polyoxyethylene alkyl ether) is used (see, for example, JP-B-21532/1977). It is known that polyoxyethylene alkyl ethers are lower than polyoxyethylene alkyl phenyl ethers in decomposition temperature, therefore readily dissipate on thermal decomposition in many cases, are less likely to remain in polymers, and form films which are apparently advantageous in yellow index of plastics (ASTM D-1925-63T) measured.
Polyoxyethylene alkyl ethers, which contain no benzene ring in the structure, do not convert into a harmful organic aromatic compound (such as benzene, toluene or xylene) on thermal decomposition, giving rise to no air pollution. Thus, the aqueous polytetrafluoroethylene dispersion comprising a polyoxyethylene alkyl ether has various advantages and yet is not in wide use because in the field of fibrous base materials impregnated with aqueous polytetrafluoroethylene dispersions wherein the dispersion is used most frequently, the following properties are required of the dispersion.
(1) For impregnation, the dispersion is relatively stabilized in viscosity and low in viscosity-temperature dependence.
(2) The dispersion has a relatively low viscosity of 10 to 30 cp at room temperature (25.degree. C.) and smoothly penetrates into fibrous base materials.
(3) When the dispersion is repeatedly applied in layers, the resulting coating still remains free of cissing or coagulation.
JP-B-21532/1977 mentions nothing about such impregnation.
We have conducted intensive research on the invention of JP-B-21532/1977 and found that this invention is characterized by using as surfactants two kinds of polyoxyethylene alkyl ethers, i.e., one having a cloud point of up to 45.degree. C. and the other having a cloud point at least 10.degree. C. higher than the former and not lower than 50.degree. C. Examples of the invention indicate that the ethylene oxide content is as high as 73.5 to 82 wt. % to give marked hydrophilic properties, such that when the composition is applied repeatedly, cissing occurs.
An object of the present invention is to provide an aqueous polyoxyethylene alkyl ether dispersion composition having excellent impregnating properties, releasing none of harmful organic aromatic compounds such as benzene into the atmosphere and less likely to cause pollution.
Another object of the invention is to provide a coated product which is diminished in the amount of residue (carbide) of a surfactant, has good hand (high degree of whiteness) and is also excellent in electric properties.
Another object of the invention is to provide a binder for use in batteries which is excellent in electric characteristics, or a coating composition for giving a clear color of high lightness.